Due to many ppl pm me to ask for the access into HG850a, here is it..
If you're using Maxis FTTH, this setup will enable you to plug the telephone into the HG850a, use custom router and you can put your Thompson router into store room or maybe rubbish bin.
You can go straight to the end of this post to see the final outcome.

Do it at your own risk.

Preparation

1. Download & install autohotkey http://l.autohotkey.net/AutoHotkey_L_Install.exe
2. After finish install autohotkey, launch it and right click on the AutoHotkey icon at the right corner taskbar, click on "edit this script", a notepad will open. Change the content to following:

3. save it and close the notepad, right click the AutoHotkey icon again and click "reload this script"
4. download putty http://tartarus.org/~simon/putty-snapshots/x86/putty.exe
5. configure your pc ip to 192.168.100.123 subnet 255.255.255.0
6. Switch off the HG850a and plug LAN cable connect ur pc to the HG850a LAN 2

Action
1. open a command prompt [windows key + R, type ' cmd ', enter]
2. type to perform continuous ping to 192.168.100.1
3. open putty client and ready for telnet: input 192.168.100.1 and choose telnet. Dont click 'Open' Yet.


4. Switch on ur HG850a and check the continuous ping
5. Once you saw you're getting Reply from 192.168.100.1.. IMMEDIATELY click "Open" in the putty client.
6. You're now successfully telnet into the HG850a but dont be happy, your access will soon be block once the HG850a finish loading the iptables, so you need to perform the following steps quick and accurate.
7. login by
username: root
password: admin

8. press the hotkey [ ctrl + alt + z ] , wait for the command to finish typing,
then immediately repeat pressing [ctrl + alt + x], let the command finish typing between the interval.This is the Hotkey that we configure into the AutoHotkey previously. This is to let the AutoHotkey help us to type the command in a very short time, before the iptables kicks in and repeatedly wipe the iptables.

9. After around 10 seconds, if you still get response from the telnet (respond to your "enter"; press enter can go next line), then you're safe for now.. but not permanent, if u power off and power on your HG850a, you will need to do from step 1 again.

Configure to have permanent access and no need to hassle so much, perform the following:

Web Interface
1. Open your browser, go into http://192.168.100.1
2. Login with admin account:
Username: telecomadmin
Password: hs5711Bbfh
3. go to 'maintain' > 'device' > 'configure file' > 'Download Configuration File' to backup the config.


4. after download, make a duplicate copy of your config so that if anything goes wrong you still have the original config.
5. open the 'downloadconfigfile.conf' using notepad
6. ctrl+f to find

7. change that line to 

8. save it and go to the HG850a web interface, go to 'maintain' > 'device' > 'configure file' > 'choose file' > select this modified config > 'Upload Configuration File'
9. After finish restore, power off and on the HG850a, you should now able to access the web interface and telnet even after power cycle.

Setup VoIP into HG850a [Maxis]
1. Go to 'Basic' > 'WAN' and you should now see by default '1_VOIP_R_VID_400' {not too sure about the name but I know there is a default there}
2. click on it to select, change the VLAN ID to 822. Change and save it as below


3. Go to Status > 'Device' > 'WAN' you should see it connected with an ip address


4. Go to 'Basic' > 'VoIP' and setup the SIP as follow


5. Go to 'Status' > 'VoIP' , you should see the status 'Registered' and the 'voip' led on the HG850a should lid up.


6. Plug your telephone into 'Tel 1' Port and try to make a call to your mobile phone.

Setup custom router
Any router
Refer to bro thankyou!
vlan capable router


Final look..


Credit: Thanks MrSpeedy for giving me some important information which originated from yjwong.